1. Field of the Invention
When using a battery operated device, it is often important to know when the useful life of the battery is about to end. For example, failure to indicate a low battery condition of a battery operated computer could result in data loss if the remaining power is insufficient to save the information and exit the application. As another example, knowledge of the remaining battery capacity of a battery operated medical device could be crucial in a medical emergency.
Batteries are depleted through use. Simply tracking use time or charge transfer is an insufficient measure of remaining battery capacity, however. Batteries lose charge over time even when not in use. In addition, the battery's temperature can affect its capacity and useful life. Furthermore, monitoring the electrical characteristics of a battery, such as voltage or resistance, often does not provide enough information regarding the capacity of the battery.
This invention relates to a method and apparatus for detecting and indicating a low battery capacity condition for a battery operated device.
2. Description of the Prior Art
Battery capacity detectors and indicators are known in the art. Most prior art battery capacity indicators are based on one or both of the following two methods: (1) measurement of a battery parameter, and (2) cumulative measurement of battery output. The measurements are typically made in absolute, not relative, terms. The prior art battery capacity indicators also generally include some way to compensate for the environmental effects of time and temperature.
The apparatus disclosed in Renirie U.S. Pat. No. 4,259,639 monitors the resistance of a lithium type battery. The Renirie device is based on a theoretical model that divides the effective resistance of the battery into two resistance components. The measuring circuit shunts the first resistance component R.sub.L by a capacitance so that measurement signal pulses measure only the depletion resistance R.sub.d. According to the Renirie model, R.sub.d will begin to increase near the end of the battery's useful life.
Peled et al. U.S. Pat. No. 4,725,784 discloses a method for determining the state of charge of lithium batteries using periodic open circuit voltage measurements and predetermined calibration curves relating the measured voltage to residual charge on the battery. Each set of test load, test load time, and recovery time parameters define a different calibration curve for a given ambient temperature. The battery's residual capacity is displayed by a set of LED's.
Bates U.S. Pat. No. 5,162,741 discloses a battery energy monitor that measures the current delivered by the battery and estimates the remaining battery energy using a temperature compensation factor to adjust for battery capacity variations with temperature. Remaining battery life is shown on a visual display.
Other battery tracking devices are shown in Oogita U.S. Pat. No. 5,065,084 and Sloan U.S. Pat. No. 5,130,659.